Various different communication systems adapted to provide wireless communication between two or more stations are known. Wireless communication media may be provided between a station of a communication network and a user equipment. Wireless communication media may also be provided between two user equipment or between two stations of a communication network.
A wireless communication systems may be used for various types of communication, such as for voice communication or data communication. A wireless system may provide circuit switched or packet switched services or both. In packet switched services data (e.g. speech data, user data, video data or other data) is communicate in data packets. The development in the wireless communication has lead to systems that are capable of transporting data in substantially high data rates i.e. the so called high speed data (HSD).
An example of wireless communication systems is a cellular communication system. In a cellular system the user equipment may access the communication network via access entities referred to as cells, hence the name cellular system. The skilled person knows the basic operational principles and elements of a cellular network and these are therefore not explained herein in any greater detail. It is sufficient to note that a cell can be defined as an radio access entity that is served by one or several base stations (BS) serving user equipment (UE) via a wireless interface therebetween. Examples of the cellular networks include networks that are based on access systems such as the CDMA (Code Division Multiple Access), WCDMA (Wide-band CDMA), TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), or SDMA (Space Division Multiple Access) and hybrids thereof.
A wireless communication system is typically provided with a radio resource management function. A feature of the radio resource management is that it may continuously adjust the use of resources such as the power levels between a base (transceiver) station and user equipment associated with said base station during communication between the base station and the user equipment. Use of radio resources may be controlled for transmissions that occur from the base station towards the user equipment (downlink) and from the user equipment towards the base station (uplink). The adjustment is done in order to provide a sufficient quality and reliability for the transmission between the base station and the user equipment in various conditions and, on the other hand, to reduce power consumption and interference caused by the communication to other devices.
A user equipment may communication simultaneously with a number of base stations. FIG. 1 shows an example where a user equipment MS1 is in communication with two base stations BS1, BS2. The simultaneous communication with a plurality of base stations may occur, for example, when a user equipment is to be handed over from a base station to another base station.
The handover may be performed by means of the so called soft handover procedure. For example, in the CDMA soft handover may be used to reduce the interference caused by the user equipment. During a soft handover the transmission power of a user equipment is typically adjusted based on power control commands from a base station that request for the lowest transmission power. Each base station involved in the soft handover measures the quality of the signal from a given user equipment and sends its power control commands to the user equipment asking the power up or down. The user equipment increases its transmission power only if all base stations involved in the soft handover request for more power.
The user equipment may receive data such as control messages, user data and so on from a base station. The user equipment may receive data from more than one base station. Some of these data transmissions may need to be responded by the user equipment. The response may, for example, be an acknowledgement that the user equipment did receive the message and/or that the user equipment did accomplish a task in response to the message and/or a response to an inquiry and/or any other feedback that may be required by the base station. The following will discuss a more detailed example that relates to acknowledgements in a third generation wideband code division multiple access (3G WCDMA) system.
In WCDMA based systems the above referred high speed data may be enabled e.g. by means of the so called high speed downlink packet access (HSDPA) technology. The high speed downlink packet access (HSDPA) may include functions such as fast hybrid automatic repeat request (HARQ), adaptive coding and modulation (AMC) and/or fast cell selection (FCS). These functions are known by the skilled person and will thus not be explained in more detail. A more detailed description of these and other function of the HSPDA can be found e.g. from a third generation partnership project technical report No. 3G TR25.848 release 2000 titled ‘Physical Layer Aspects of UTRA High Speed Downlink Packet Access’. It shall be appreciated that although the HSDPA has been specified for use in the WCDMA, similar basic principles may be applied to other access techniques.
At the present it is assumed that in the high speed downlink packet access (HSDPA) each user equipment receiving data on a high speed downlink shared channel (HS-DSCH) also has an associated dedicated channel (DCH) allocated. The dedicated channel may be mapped to a dedicated physical channel (DPCH) in the physical layer. The DPCH is typically divided into dedicated physical data channel (DPDCH) and dedicated physical control channel (DPCCH) both in the uplink and the downlink. Data such as the power control commands, transport format information, and dedicated pilot symbols are transmitted on the DPCCH. Information such as diversity feedback information may also be transmitted on DPCCH in the uplink. The HS-DSCH may be mapped to one or several high speed physical downlink shared channels (HS-PDSCH) in the physical layer.
The associated dedicated channel is typically provided both in the downlink and the uplink. The dedicated channel is typically used to carry HSDPA related information/signalling as well as other dedicated data such as speech and control data. The user equipment may communicate with several base stations at the same time. For example, the associated dedicated channel may be in soft handover.
In addition to associated dedicated channels, the HS-DSCH may be associated also with a shared control channel (SCCH). The SCCH can be used to carry HS-DSCH specific information/signalling to those users receiving data on the HS-DSCH.
A current proposal is to use the dedicated channel to inform the user equipment that it has data to be read on the HS-DSCH and SCCH. That is, only those users receiving data at a given time will receive an indication on the dedicated channel. The dedicated channel may be called as a pointer channel since it points to the shared channels. The dedicated channel may also contain information about modulation and coding schemes, power levels and similar parameters used for the shared channels. This information can be sent also on the shared channel. The shared control channel on the other hand is used to carry information that is specific to the data transmitted on the shared data channel (HS-DSCH). This information can contain for instance packet numbers for the HARQ and so on. The shared control channel can be sent on a separate code channel (code multiplexed) or using the same code channels as HS-PDSCH (time multiplexed).
Unlike the dedicated channel, the HS-DSCH is assumed not to be in soft handover. That is, each base station is assumed to have their own shared channel and the user equipment is assumed to receive data from only one base station at a time. The so called fast cell selection (FCS) technique may be used to switch the data transmission from one base station to another. However, the shared channels does not use power control. Instead, the shared channels are proposed to be transmitted with fixed or semi-fixed power. The term ‘semi-fixed’ means in here that the power is not changed often. The power could, for instance, be a cell specific parameter.
In the currently proposed arrangements the high speed downlink shared channel (HS-DSCH) is planned to be associated with a dedicated channel which would carry in the downlink at least information regarding the timing when the receiving station is to receive on a shared channel. The associated dedicated channel may possibly carry also other information. In the uplink, the associated dedicated channel may carry, for example, the required acknowledgements (ACK) for a fast HARQ.
The inventor has found that this way be problematic for example in the context of the uplink power control of the fast HARQ acknowledgements. A problematic situation may occur especially when the associated dedicated channel is in the soft handover mode. During the soft handover the uplink power is adjusted in accordance with the best quality uplink among an active set of base stations. However, signalling on the high speed shared channel may be transmitted from another base station. The communication link between the user equipment and said other base station may be of poorer quality than said best uplink connection. Nevertheless, said other base station expects to receive responses such as an acknowledgement from the user equipment. Since the quality of this uplink connection may be of substantially poorer quality than what the best uplink is, there is a risk that the response is not properly received and decoded or is not received at all.
The fast cell selection function may be used to guarantee in some occasions that best possible downlink is utilised for communication towards the user equipment. However, the base station that provides the best uplink may be different than the base station providing the best downlink. This may be so, for example due to fast fading or other changes in the signalling conditions. This may increase the unreliability of the responding functionality.
The prior art proposals for solving this problem include so called strong coding, e.g., by using repetition coding. In the repetition coding the acknowledgement (ACK) bit or bits is/are repeated several times. This, however, may cause too much additional load on the air interface and/or reserve too much of the radio resources if one wants to guarantee the correct reception of the acknowledgement message.
Another prior art proposal is the so called fixed power offset for acknowledgement (ACK) transmissions. This means that all acknowledgement messages are transmitted with increased or a certain power in order to ensure that the acknowledgement is received even through the poorest quality uplink. However, even a fixed power level for acknowledgement messages may not completely solve the problem of substantially poor signalling conditions. Thus situations where the power is not high enough may still occur. On the other hand, it is also possible that the fixed power level is unnecessarily high. Thus, in addition to reliability problems this approach may be disadvantageous in that too high power is used by the user equipment causing interference and unnecessarily high power consumption.